Sound reproduction in a motor vehicle presents a certain number of specific problems. Those of these problems which belong to the electro-acoustic domain can be classified in two categories: on the one hand, those of an ambient noise type and, on the other hand, those associated with the sound balance. In all the known devices for sound reproduction, these two problems are dealt with independently of one another.
The state of the art relating to the compensation of sound volume as a function of the ambient noise can be illustrated by French Patents Nos. 2,435,879, 2,466,901, 2,513,839 and 2,529,727.
The state of the art relating to the sound balance is much less extensive and can be illustrated by French Patent No. 2,514,213 in the name of the Applicant.
FIG. 1 shows the basic diagram of a device which is commercially available under the name "S-TRONIC MK3" and which represents the most sophisticated state of the art with respect to an amplifier for a motor vehicle.
The signal input E is followed first by an equalizer 1, followed by a controlled-gain amplifier 2 which, in turn, is followed by a power amplifier 3 feeding a loudspeaker 4. The equalizer 1 comprises a control device 5 including a certain number of preset potentiometers. The controlled-gain amplifier 2, for its part, is controlled by a circut 6 which produces a control voltage from respective signal levels originating, on the one hand, from an ambient-noise microphone 7 and, on the other hand, from the modulation taken downstream of the controlled-gain amplifer 2 across a preset potentiometer 8.
The different automatic volume control devices differ essentially in the configuration of the circuit 6 which may be more or less complex in order to account for various parameters.
However, whatever the configuration of the circuit 6, a basic function must always be provided and this is used to estimate the level of the ambient noise. This estimation is effected by comparing the respective levels of signals produced by the microphone 7 and consisting of noise produced by the vehicle and of the signal produced by the loudspeaker, and of the pick-up 8 containing only the signal. This estimation is made with a dB error given by the formula: EQU dB=dS.times.S/B
in which dS is the non-linearity error of the electroaccoustical chain from local loudspeaker to microphone, and S/B is the ratio between useful signal and ambient noise.
The invention has the objective of implementing an audio amplifier for a motor vehicle, which amplifier eliminates the preceding disadvantages by providing simultaneous automatic control of volume and sound balance in an efficient manner and without multiple and delicate control systems.